This invention relates to electromechanical energy conversion devices, circuits that condition and control their electric power, means to detect and process their control signals, and composite configurations thereof that provide controlled motor and generator action as variable-speed regenerative brushless motor and controller systems.
Variable-speed brushless electric motor systems are known in the art that provide the high starting torque and control attributes of commutator motors without sliding commutators and their need for periodic maintenance, their friction, sparks which could be hazardous in explosive atmosphere, and their broadband radio interference. They are comprised of motors with polyphase wound stators that are provided variable-frequency commutated drive by power semiconductors, with various rotor types for synchronous and/or asynchronous operation.
Some brushless motor systems effect stator commutation by sequential switching of the stator windings, synchronized to the rotor angle or speed, which essentially causes the stator magnetic field to rotate stepwise. A practical limit to the number of stator windings and switching elements results in torque pulsations due to switching discontinuities, along with power loss and motor heating due to harmonic and reactive current. Also, tandem current regulators are required.
Other known systems incorporate switching regulators to generate sinusoidal polyphase stator voltages at variable amplitude and frequency; that variously include four-quadrant multipliers, nonlinear function generators, and phase or frequency compensation; with critical matching and adjustment of system parameters required to achieve reasonably acceptable power factor or slip frequency. Motor systems ranging from medium to high power incorporate thyristor switching regulators such as the McMurray inverter.
In principle, such systems circumvent the aforementioned torque pulsations, power losses, and heating of stepwise commutation. Moreover, current regulation can be accomplished by the same power switches which generate the polyphase voltages, so tandem regulation is not required.
However, if the intended sinusoidal polyphase currents from the inverters are substantially distorted, the possible advantages of such systems cannot be fully realized. Said distortion results from the motor voltage reaching that of the DC supply to the inverter, causing harmonic power losses in the motor and ripple current fed back to the supply line at relatively low frequencies which are dependent on motor speed and consequently not feasibly absorbed by power filter circuits. Additionally, slow thyristor switching necessitates compromises between power filter size vs. harmonic power losses in the motor vs. increased switching losses and unreliable commutation, and also restricts applications to systems with limited rotational speed. Furthermore, motors are sometimes reversed by operators while rotating at high speed; and unless the system incorporates safeguards for such operation, excessive power can be dissipated in wasted energy that may cause damage to the system. Also, the well known regenerative braking properties of the electric motor cannot be realized unless appropriate circuit conditions are provided.
Motor systems that are not subject to the aforementioned drawbacks and limitations would afford improvement to numerous useful applications, such as motive power for electric cars, buses, and trucks, mobile lifts and miscellaneous conveyances, servomechanisms in a wide range of sizes, motive devices for physically handicapped persons, and a wide variety of machine tools; and would facilitate broader application to systems requiring high rotational speed and negligible rotor heating, such as hermetically sealed and evacuated electrically coupled flywheel energy storage systems.
Therefore, an objective of this invention is to provide improved brushless motor and controller systems, with high starting torque and efficient drive and regenerative braking over a broad speed range, without torque discontinuities, with negligible power loss due to harmonic and reactive current in the motors, and especially negligible rotor losses.
Another objective is to reduce the size, power loss, and cost of reactive power filters for such motor and controller systems, and yet further reduce current and voltage ripple and peaks in the motors, controller circuits, and supplies.
Another objective is to provide reliable systems that do not require critical compensation, adjustment, and matching; so that unity power factor (and thus maximum torque for given currents and negligible losses from reactive currents) is inherently maintained over the entire motor speed and torque range without said compensation, adjustment, or matching.
Another objective is to provide motor and controller systems with adaptive control means that respond to all operator commands (including reversal at high speed, and full torque demand with locked rotor) effectively and efficiently, with inherent protection from damage.
To achieve these objectives, this invention provides circuit and system configurations that embody the following fundamental principles:
Firstly, this invention is based on the fundamental principle that orthogonal components H.sub.x = H.sub.m cos.theta. and H.sub.y = H.sub.m sin.theta. combine to form a resultant with magnitude H.sub.m and direction .theta.. Referred to a 2-phase, single pole-pair motor with synchronous rotor, orthogonal components H.sub.x and H.sub.y respresent the respective magnetic field intensities of orthogonally positioned stator windings with respective currents I.sub.m cos.theta. and I.sub.m sin.theta., produced by systems means wherein .theta. is the rotor field angle relative to the stator. Interaction of the resultant stator field and the rotor field causes a torque proportional to their product which tends to align them. Since the system is arranged so the resultant stator field is always displaced 90.degree. from the rotor field, the motor operates at a 90.degree. torque angle and unity power factor.
This principle is readily seen to be applicable to all polyphase synchronous motors with N.sub..phi. phases of two or more and N.sub.p pole-pairs of one or more, wherein the resultant stator field is displaced 90.degree. /N.sub.p mechanically from the rotor field.
Secondly, this invention is based on a fundamental principle of switching regulators, supplied with constant Dc voltage V.sub.dc and controlling balanced and undistorted sinusoidal polyphase currents at unity power factor so that total power P.sub.t is exchanged: The sum of the currents, averaged over several switching cycles, to or from said plurality of switching regulators, is constant and approximately equal to P.sub.t /V.sub.dc. Pulse currents from each switching regulator can be characterized by ripple components at the power switching frequency f.sub.s, its sidebands f.sub.s + 2 N.sub.p .theta. and f.sub.s - 2 N.sub.p .theta., harmonics 2f.sub.s 3f.sub.s, etc., and their sidebands, wherein N.sub.p .theta. is the electrical frequency of rotation. The lowest frequency component of ripple current is near f.sub.s. Selected frequency components of the summed currents can be reduced by time-staggering or phasing such pulse currents, so that their sum is appreciably less than that resulting from non-staggered timing. Insofar as the components near f.sub.s are the lowest frequency and largest amplitude, it can be seen that optimum time-staggering can be achieved by 180.degree. relative phasing of pulses from two summed sources, 120.degree. with three, 90.degree. with four, etc., and 360.degree. /N.sub..phi. N.sub.p in general.
Accordingly, a motor and controller system is herein described that achieves inherent unity power factor polyphase operation, that maintains undistorted sinusoidal stator currents over its entire broad speed range, that achieves time-staggered power switching to control said currents at rates 10 to 100 times the practical limit of known inverters for motor control systems, and achieves a ratio of effective sinusoidal voltage to ripple voltage which is double that of known inverters for motor systems.
Furthermore, system configuration and control logic means are described that insure optimum system response to all external commands regardless of existing motor speed and direction or mechanical load.